Evidence-appraisal glossary
Hazard ratio
Hazard ratio (HR) compares how quickly an event happens in two groups over time. It is the ratio of their hazard rates, the instantaneous chance of the event among those still at risk. An HR of 1.0 means no difference; below 1.0 means slower events in the treated group, above 1.0 means faster.
Also called: HR.
A hazard ratio comes from survival analysis, which studies both whether an event occurs and when. The hazard is the rate at which events happen at any given moment among people who have not yet had one; the hazard ratio divides the treated group's hazard by the control group's. An HR of 0.75 means the event is happening about 25 percent less often per unit of time in the treated group. It is not the same as a relative risk, and it does not tell you how much longer someone goes event-free, only the relative rate. When reading a study, check the confidence interval and whether it crosses 1.0, and look for evidence that the proportional hazards assumption holds, meaning the ratio stays roughly constant over the follow-up. For example, a cancer trial reporting an HR of 0.70 for death indicates a lower rate of dying over the study, but the absolute survival difference and median follow-up time still matter for interpreting it.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.